This invention relates to aluminum cathodes for electrolytic capacitors. More particularly it relates to a high purity aluminum cathode for electrolytic capacitors made by depositing aluminum on a porous high surface-area dielectric spacer.
Aluminum cathodes are used in both aluminum and tantalum foil wound electrolytic capacitors. Generally, the aluminum foil used for cathodes is of a lower purity (less costly) than that used for anodes. In some instances, after the etching of lower purity cathode foil, residual copper is left on the surface. There are processes that have been devised for removing this residual copper, e.g., washing the etched foil in nitric acid. However, there are applications in which high-purity cathodes are required, and, generally, high-purity foil has been necessary to make the cathodes for these applications.
Aluminum anodes and cathodes for capacitors have been produced in the prior art by spraying high-purity aluminum on calendered kraft paper or gauze. Metallized paper and plastic films have also been used in capacitors. Sprayed particles are irregular, producing rough and uneven coatings that are unsatisfactory for the present application, inasmuch as sprayed layers are too coarse, will not conform precisely to the surface of a porous paper, and thus do not have the high cathode capacitance required.
Prior art metallized papers and films are smooth, and the metallization is too thin, particularly those with self-healing characteristics that typically have 200-400 A thicknesses. Those thicknesses do not provide the high surface conductance required for the present invention.
The cathode material of the present invention would not be used in a self-healing capacitor, as the deposited metal is too thick. An important factor is that the prior art metallized papers and films themselves are not porous enough and are too smooth to provide the convoluted, high surface-area required for this invention.